1. Field of the Invention
The present invention relates to a temperature control structure for batteries and a battery box for housing such batteries, and more particularly to a temperature control structure for uniformly and effectively controlling the temperatures of respective batteries which are housed in arrays in a battery box, and a battery box for housing such batteries.
2. Description of the Related Art
It has heretofore been confirmed that the performance of batteries varies depending on the temperature of the batteries. For example, the mileage and durability of batteries mounted on an electric vehicle vary depending on the temperature of the batteries. It is therefore desirable for the batteries on such an electric vehicle to be used in a predetermined temperature range in order to maintain their mileage and prevent their durability from being reduced.
There has been proposed an apparatus for cooling batteries for use on an electric vehicle as disclosed in Japanese laid-open patent publication No. 5-169981. In the disclose cooling apparatus, a total of twenty batteries arranged in rows and columns are housed in a battery box, and air introduced from a front wall of the battery box is guided to flow backward through the battery box to cool the batteries placed in the battery box. The air is caused to flow backward through the battery box by air discharge fans that are mounted on a rear wall of the battery box.
A conventional battery box and a cooling apparatus thereof, which is operated on the same principle as that of the proposed cooling apparatus, will be described below with reference to FIGS. 1 and 2 of the accompanying drawings.
FIG. 1 shows in schematic plan the positional relationship between a battery box, batteries housed therein, and cooling fans for cooling the batteries. FIG. 2 shows in schematic vertical cross section the battery box, the batteries, and the cooling fans which are illustrated in FIG. 1. As shown in FIG. 1, three fans 4a.about.4c are mounted on a front wall of a battery box 2 for forcing cooling air into the battery box 2, and two air discharging fans 6a, 6b are mounted on a rear wall of the battery box 2 for drawing air out of the battery box 2. A total of twenty batteries 8 arranged in four rows and five columns are housed in the battery box 2.
When the fans 4a.about.4c and the air discharging fans 6a, 6b are operated, cooling air introduced into the battery box 2 by the fans 4a.about.4c flows backward along the batteries 8 as indicated by the arrows, and is finally discharged out of the battery box 2 by the air discharging fans 6a, 6b. While flowing in the battery box 2, the cooling air can cool the front surface, two side surfaces, and rear surface of each of the batteries 8.
FIG. 3 of the accompanying drawings illustrates the temperature of the cooling air as it flows in the battery box 2. As the cooling air flows to the right in the battery box 2, the temperature of the cooling air gradually rises by absorbing heat from the batteries 8 which the cooling air has contacted, and hence the ability of the cooling air to cool the batteries 8 is gradually lowered. Consequently, the batteries 8 are subject to various temperatures which differ in the downstream direction of the air flow.
FIG. 4 of the accompanying drawings shows some of the batteries 8 in fragmentary plan. Each of the batteries 8 comprises an array of six cells 8a. Since each battery 8 is cooled at its front surface, two side surfaces, and rear surface, the first and sixth cells 8a are cooled more strongly than the other cells 8a positioned therebetween because the first and sixth cells 8a have much more surface area for contact with the cooling air than the other cells 8a.
With the above cooling apparatus, therefore, the batteries suffer large temperature differences, and the cells in each of the batteries also suffer large temperature differences.
The large temperature differences between the batteries result in different battery charging efficiencies, which allow overly charged and insufficiently charged batteries to exist at the same time in the battery box. When those batteries are repeatedly charged and discharged, therefore, some batteries or cells end their service life more quickly than the others. Since the batteries are used and replaced as a set, the service life of the set of batteries tends to become far shorter than an expected period of time.
If the temperature of some batteries or cells is lower than a given temperature of 15.degree. C., for example, then their chargeableness and dischargeableness are lowered. Stated otherwise, a battery cannot perform its desired functions at temperatures lower than a predetermined temperature. Batteries whose bottoms are held in contact with a battery case allowing the heat of the batteries to be radiated outwardly through the battery case may not stand practical use in winter or cold climates where the temperature around the batteries is low.
The fans 4a.about.4c and the air discharging fans 6a, 6b which are associated with the battery box 2 as shown in FIGS. 1 and 2 are usually combined with respective electric motors for rotating the fans. Accordingly, the conventional battery temperature control structure is relatively expensive to manufacture and heavy.
The electric motors for rotating the fans are required to be connected to respective on/off switches and respective fuses. Consequently, a complex electric circuit is required to control the electric motors, resulting in a further increase in the cost of the conventional battery temperature control structure.